In silicon photonics (SiP), most high-speed devices are designed using PN-junctions. The variation of the carrier density within the waveguide, which can be controlled by varying the applied voltage, modifies the effective index of the optical mode. Thus, a PN junction can be used to create a high-speed optical phase modulator (PM) which is also often incorporated into a Mach-Zehnder Interferometer (MZI) to create an amplitude modulator. However, as described by Soref's equations (such as described in R. Soref and B. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron., vol. 23, no. 1, pp. 123-129, 1987, the contents of which are incorporated by reference), charge carriers not only modify the silicon index of refraction but also its absorption coefficient. As a result, SiP PMs have a significant impact on the amplitude of the transmitted wave as well. Thus, a substantial amount of undesired Residual Amplitude Modulation (RAM) is added to the optical signal. A PM can be used to encode data in both the phase and the amplitude of an optical signal. If the information is encoded directly in the phase, the RAM is a power instability that can decrease the performance of data transmission. If the information is encoded in the amplitude (e.g., using a PM in an MZI interferometer), the RAM will be converted in chirp which is also damaging for telecom system.
Phase modulators are inherently imperfect components in the fact that they always produce some amount of amplitude modulation as well. For example, lithium niobate (LiNbO3) modulators are generally accompanied by a residual amplitude modulation (RAM) of a few tenths of a percent. However, in the silicon photonic technology, phase modulation is typically realized using a PN-junction operating either in a depletion-mode (carrier depletion mode) or a forward bias mode. In both cases, free-carriers are being modulated, which leads to a RAM of several percent, about 10%/rad, thus degrading a lot the performance of systems making use of such phase modulators, and taking away the benefits of using low-cost silicon photonics chips to realize those systems.